Locate the format option and give that a try. The beeping is a indicator that their is a possibility the hard drive if faulty. Format the hard drive as suggested or replacement is required. Once you have rectified the hard drive with format or replacement the beeping will be silenced.
Replacing the HDD is a no-brainer.
Purchase survillance-rated HDD (like WD Purple) of size in TB that your NVR is compatible with.
Unplug NVR.
Open case cover, screws on side, maybe 2 at upper rear.
Unplug the data and power cables to the HDD.
Remove the screws holding the HDD to the case.
Swap the old HDD with the new and re-fasten with screw, re-install the data and power cables.
Replace cover.
Plug in NVR and power up.
Go to Settings => Maintenance or similar and format the HDD.